1. Field of the Invention
This invention relates to a plastic valve assembly and more particularly to a butterfly-type valve having a plastic body portion with a valve seat formed by a metal sleeve molded within and sealed to the plastic valve body with an annular rubber sleeve insert vulcanized to the metal sleeve and arranged to receive a rotatable valve disc movable into and out of sealing engagement with the rubber sleeve insert to close and open the valve assembly.
2. Description of the Prior Art
Butterfly valves are well known in the art and conventionally include a valve housing or body portion within which an annular elastomeric seat is mounted for the purpose of effecting a resilient seal against a pivotal valve disc. The disc is rotatable from an open position, aligned with the direction of fluid flow through the valve, to a closed position at right angles to the direction of flow. In the closed position, the peripheral edge of the disc tightly engages the valve seat to effect a seal. In the conventional butterfly valve, the valve seat is molded of an elastomeric material to provide it with a limited degree of resilience. When the valve disc is rotated to a closed position, it abuts the valve seat in an interference fit to effect a fluid tight seal between the valve disc and the valve seat.
The need for a certain degree of resilience, and thus displaceability of the valve seat necessitates the use of an elastomeric material. Materials of this type have a tendency to creep or migrate when subjected to high pressure, particularly when the pressure is applied to the seat on one side of the disc without a corresponding supporting pressure on the other side of the disc. The need to control this migration or creep without sacrifice of the necessary resilience of the elastomeric material has caused the industry to adopt the use of reinforcing members in the seat. These reinforcing members are rigid and provide support for the elastomeric valve seat limiting its ability to creep or migrate.
One arrangement for reinforcing the elastomeric valve seat is an annular ring or band of rigid material embedded within the seat as the seat is molded. It is important that this reinforcement be accurately positioned within the seat if the seat is to have uniformity of strength and accurate control of deflection, migration, and creep is to be maintained. During the process of molding the valve seat, the elastomeric material is injected into the mold at high pressures. This tends not only to deflect but also to displace the reinforcement member and thus mislocate it within the seat. This results in a defective seat.
U.S. Pat. No. 3,537,683 attempts to resolve this problem by modifying a conventional mold for a valve seat by the addition of lateral pins. With this arrangement openings are provided in the valve seat which extend from the outer periphery of the valve seat through the reinforcement member. However, this arrangement is known to result in leakage under certain circumstances requiring that the openings be plugged to prevent leakage. Even if plugging is not effective, an opening is created through the reinforcement member thus permitting differential deflection of the seat at the hole because the plug cannot provide the same uniformity of support as would a continuous, uninterrupted reinforcement member.
An example of another approach to solving the problem of reinforcing the valve seat to prevent creeping is disclosed in U.S. Pat. No. 3,595,523. There a rubber valve seat is backed up by a plastic retaining ring which is mounted to the plastic housing by screws. Molded reinforcement members for conduits are also known. In U.S. Pat. No. 3,526,386 a T-connection for a piping system is disclosed and includes an annular body portion and a pair of coaxially aligned tubular branches. The body portions are fabricated of plastic material, and a metallic sleeve is molded within each tubular branch when the valve body portions are formed to reinforce the branch.
U.S. Pat. Nos. 3,241,806; 3,502,299; 3,589,678; 3,647,180; 3,667,726; 3,771,764; 4,006,882; 4,025,050 and 4,036,468 disclose butterfly valves having metallic body portions with composite valve seats formed of various materials, i.e., metallic and nonmetallic.
It is also know in the art to provide a metallic butterfly valve with a metallic ring secured to the metallic valve housing with a resilient liner retained within the metallic ring. In U.S. Pat. No. 2,923,524 the retaining ring is formed by a plurality of arcuate segments. In U.S. Pat. No. 3,473,784 a Teflon liner is supported in the valve housing by a steel ring. In U.S. Pat. No. 3,525,499 a stainless steel ring is fused to the metal valve body by casting. U.S. Pat. No. 3,612,483 discloses a rigid metal Stellite seat bonded to a valve body.
In U.S. Pat. No. 3,940,108 a reinforcement ring is disclosed for a butterfly valve having a metallic body portion. The valve seat is a molded member formed by an annular, rigid reinforcement ring embedded in a body of molded elastomeric material. Support projections are provided on the reinforcement ring to support the ring accurately in place within a mold while the elastomeric material is injected into the mold and surrounding the ring.
Still further butterfly valves are disclosed and described in the following U.S. Pat. Nos. 3,311,128; 3,447,780; 3,633,872; 3,675,888; 3,782,684; and 3,790,130.
While it has been suggested to provide reinforcing members for the elastomeric seat ring for a butterfly valve, the known arrangements have been considered ineffective to develop a seal directly on a valve body of polyethylene. Therefore, there is need for a butterfly valve fabricated of a plastic material in which a fluid tight seal is maintained between the valve body and the valve closure member and is not subject to creeping of the polyethylene valve body from the valve closure member producing a radial gap around the closure member resulting in leakage of the butterfly valve when the valve closure member is moved to a closed position.